Automatic gain control



April 11, 1939. PH 6, F RN AM 2,153,747

AUTOMATI C GAIN CONTROL Filed Feb. 3, 1955 Patented Apr. 11, 1939 UNITED STATES PATENTv OFFICE AUTOMATIC GAIN CONTROL Delaware Application February 3, 1933, Serial No. 655,117

1 Claim.

This invention relates to radio receiving systems employing automatic gain control and particularly to arrangements adapted to suppress the development of automatic gain control operation for radio inputs below a predetermined level as well as to suppress all output from the receiving system for radio inputs below a predetermined level.

There are many cases in which it is feasible to operate automatic gain control systems by using as a gain control biasing voltage the direct current output from a diode rectifier initially biased to prevent its operation for carrier inputs below a predetermined level. At the same time, certain disadvantages occur with this type of control, of which the most serious is a distortion of the modulation envelope and a consequent distortion of the amplified audio output. Various methods for overcoming this distortion have been proposed, some of which are described and claimed in my copending applications Serial Nos. 524,383 and 611,402, filed March 21, 1931 and May 14, 1932, respectively, Patent Nos. 2,096,393 and 1,953,554, dated October 19, 1937 and April 3, 1934, respectively. Circuit arrangements adapted to suppress all receiver output for radio inputs below those at which the receiver noise becomes objectionable have been associated with automatic gain control systems, and often involved the use of one or more extra vacuum tubes. My copending application Ser. No. 641,280, filed Nov. 4, 1932, Patent No. 2,003,110, .dated May 28, 1935 describes and claims a control system in which one vacuum tube may perform the functions of detection, audio amplification, automatic gain control, and automatic noise suppression, but this arrangement may produce some distortion of the modulation envelope due to the action of the biased diode rectifier which supplies the gain control bias.

It is an object of this invention to provide a radio receiver system of the noise-suppression, automatic gain control type which will eliminate this source of distortion. An object is to provide a combined detector-audio amplifier-automatic control stage of the noise-suppression type and which performs the function of automatic gain control. without introducing any distortion of the modulation envelope. A further object is to provide an automatic gain control system including a current limiting element and circuit elements for controlling the operation of the current limiting element as a function of the strength of the received carrier energy. Another object is to provide, in a radio receiver, an automatic gain control system in which the direct current output from a diode detector is amplified and the resulting voltage, either directly or alternatively through an intermediate delaying circuit, is employed as the gain control bias. A further specific object is to provide a detector-amplifiercontrol stage, including a single tube and associated circuit elements, for performing the several functions of a biased or noise-suppression diode detector, audio frequency and direct current amplifier, and delayed automatic volume control, the control system including diode elements upon whichno radio or audio frequency voltages are impressed.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawing in which the single figure is a fragmentary circuit diagram of one embodiment of the invention.

In the drawing, the reference numeral I identifies a radio frequency amplifier, including one or more vacuum tubes, which receives carrier wave energy from an appropriate collector structure and which works into a detector and audio amplifier or other load circuit. The exact design and construction of the radio amplifier are not, except as hereinafter noted, a part of the present invention, and only one amplifier tube 2 is illustrated.

The output circuit 3 of the last radio amplifier stage is coupled to the tuned input circuit 4 of the detector-amplifier-automatic control stage of the receiver, and establishes a radio input voltage E on the detector. The single tube 5 of the combination stage may be a tube known as Type 55, which includes a cathode K, a pair of anodes A1, A2, a control grid G and a plate P.

Referring first to the circuit elements which cooperate with the tube 5 to give the stage the function of a detector and an audio frequency amplifier, the high potential terminal of the tuned input circuit 4 is directly connected to the anode A1, and the opposite terminal is connected to the cathode K through an audio frequency resistance 6 and an audio frequency by-pass condenser 1, the resistance 6 being by-passed for radio frequency by a condenser 8. The audio frequency voltage developed by diode rectification across the resistance 6 is impressed upon the control grid G through a radio frequency filter comprising resistance 9 and a radio frequency condenser ID.

The general arrangement of the stage is similar to that illustrated in Fig. 2 of application Ser.

No. 641,280, in that the plate P is .directly connected to the source of plate current, indicated as +B, and the output resistance II is connected, through a condenser l2, to the movable contact of a switch l3 which may be connected to the grid G or, alternatively, to the cathode K. To regulate the output level, an adjustable tap M is slidable along resistance H to impress upon. the succeeding audio load circuit, not shown,any desired portion of the audio voltage developed across resistance II. The plate is by-passedto ground for audio frequency currents by the condenser l5, and the entire audio frequency volt-' ode K of the gain-controlled tube or tubes 2 through a biasing resistance 22, and to the anode A2 of tube 5 through a resistance 23.

The described connection of the cathode K of tube 2 to a point of positive potential constitutes one essential distinction between the present invention and that described in application Ser. No. 641,280. The control grid G1 of the tube 2 is returned, for direct current potentials, through a lead 24 and an alternating current filter, comprising resistances 25 and capacities 26 to a switch arm 2'! which may connect the lead 24 either to the cathode K orto the anode A2 of the tube 5. The present circuit includes the further feature of a switch 1' for connecting the cathode terminal of resistance 6 to the resistance I 8, as shown, or, alternatively, directly to the cathode; thus short-circuiting the condenser 1.

The operation of the circuit is as follows. In the absenceof a radio frequency voltage on the input circuit 4 of tube 5, the plate current flow through the cathode bias resistance I6 is relatively high and the cathode K is maintained at a high positive voltage E1 with respect to ground. The positive potential applied to anode A1 and grid G by the connection of tap l9 to a point of positive potential E2 is less than the positive potential established on cathode K by the plate current flow. So long as the peak radio input .voltage E on the detector diode remains below put voltage E exceeds this critical value, rectification takes place and both direct current and audio frequency voltages are developed across resistance 6 and applied to the grid G. The rectified direct current voltage on grid G makes the grid potential more negative with respect to the cathode K, and thereby reduces the plate current flow. This reduces the voltage E1 across the cathode bias resistance IS. The reduction in voltage E1 removes the initial bias voltage (E1E2) on anode A1, and normal diode detection of the radio voltage E results.

The reduction in the voltage E1 across the cathode bias resistance l6 may be employed directly as an automatic gain control voltage by made more negative by the decrease in the positive potential applied to grid G1 by the potential drop across resistance Hi.

It is usually preferable, however, to employ the anode A2, in conjunction with cathode K, as a cut-off device or two-element valve for postponing the development of a gain control bias until the cathode potential has fallen so low as to remove all of the initial bias voltage (E1E3) impressed on the anode A2. This method of operation takes place when the switch arm 2! is adjusted to connect the gain control lead 24 to the anode A2. In general, a change in the adjustment of switch arm 2'! requires a change in the value of the cathode biasing resistors 22 of the controlled tube or tubes 2, or, alternatively, a change in value of the voltage E2 by an adjustment of the tap upon resistance 20. To prevent alternating current voltage of either radio or audio frequency from being developed between anode A2 and cathode K by inadvertent couplings between those elements and other circuit elements, an audio by-pass condenser 28 may be connected from A2 to K. It will be noted that the described circuit connections do not tend to establish either a radio or an audio frequency potential on anode A2 and, when the tube 5 is of such construction as to avoid substantial capacitive couplings between anode A2 and the other cold electrodes, the by-pass condenser 28 may be rendered unnecessary by so arranging the circuit elements as to avoid the introduction of couplings external to the tube. The condenser 28 or other means for preventing a radio or audio potential between anode A2 and cathode K has the effect of making the gain control bias voltage developed by direct current through resistor 23 independent of the modulation strength of the received radio wave.

With the switch arms I and 21 in their preierred positions, as shown in the figure, and remembering that the cathode potential is higher than that of the tap l9 when the radio input voltage E is zero, it is apparent that so long as the voltage E3 is less than the cathode potential, no current will flow in resistance 23, and hence no gain control bias will be developed until the radio voltage level E is increased beyond that required to produce rectification in the diode, AiK. That is to say, no current will flow in resistance 23 until the cathode potential has dropped to a value equal to E3.

If the voltage E2 is set equal to E3 this means that the carrier level E will continue to rise proportionately with increasing antenna inputs until the noise suppression bias on the diode detector A1K has been completely removed. At this level, however, the flow of current in resistance 23 will produce automatic gain control bias for a further increase in the radio input voltage E so that, for increasing antenna inputs, the voltage E will tend to stay constant.

It will be noted that the action. of anode A2 and cathode K is not that of a rectifier of radio voltages, as was the case in the invention described in my ,copending application Ser. No. 641,280, but is simply that of a unilateral conductor or two-electrode valve operating on a ,direct current input and furnishing a direct current output.

25 may be of low value, a desirable condition from the standpoint of grid current variations in the amplifier tubes being controlled. The removal of a permanently biased diode from the radio frequency system avoids the generation of excessive harmonic voltages of the input frequency which, in the case of intermediate frequency amplifiers in superheterodyne receivers, are particularly objectionable on account of the disturbing beat notes introduced by feedback into the tuned high frequency circuits.

In case it is desired to remove the noise suppression feature for any reason, such as that of realizing the full sensitivity of the receiving system for very weak inputs, the resistor 6 may be connected directly to the cathode by adjusting switch I to short-circuit condenser I and, at the same time, to open the circuit through resistor I8 to make the cathode potential E1 dependent only upon the electron current of tube 5.

The following values of the principal circuit elements were used in an embodiment of the invention which served as the second detector of a superheterodyne receiver.

Resistance 6=200,000 ohms; Resistance 9=100,000 ohms; Resistance 16=15,000 ohms; Resistance 18=50,000 ohms; Resistance 23=200,000 ohms; Resistance 25=200,000 ohms: Condenser 7:0.5 microfarad; Condenser 26:.05 microfarad; Condensers 8, 10:100 micromicrofarads; Condenser 17:1000 micromicrofarads; E2: to volts;

Es=60 volts;

+3 to B=200 volts.

While I have described a preferred embodiment of the invention in which a single tube performs,

in association with appropriate circuit elements, a number of functions, the same method of operation may be secured by the use of a plurality of tubes which individually perform one or more of the functions of the tube 5.

It will be apparent that the invention is not restricted to the particular circuit herein illustrated, and that various changes may be made in the circuit elements, and in their relative relationships and values without departure from the spirit of my invention as set forth in the following claim.

I claim:

In a radio receiver, the combination with a radio amplifier and a rectifier, of a direct current amplifier including a vacuum tube having a cathode cooperating with a plate and a control grid, a plate circuit resistance connected between the cathode and ground, circuit elements for impressing the direct current voltage output of said rectifier upon the direct current amplifier to decrease the direct current output of the amplifier as the direct current output of said rectifier increases, means preventing the development of rado voltages between the elements of said vacuum tube, and circuit connections between said direct current amplifier and said radio amplifier for transferring to said radio amplifier a gain control voltage which automatically reduces the gain of the radio amplifier as the strength of a radio signal impressed thereon increases, wherein said radio amplifier includes a tube having a cathode and a control grid, in combination with a source of direct current and means initially impressing upon the cathode andcontrol grid of said amplifier tube different positive potentials which bias said amplifier tube for high amplifi- 

